1. Field of the Invention
The present application relates to nuclear magnetic resonance (NMR) imaging, and more particularly to a method for suppressing artifacts in NMR images created by two-dimensional (2D) NMR selective excitation pulses.
2. Description of Related Art
Nuclear magnetic resonance (NMR) imaging utilizing two-dimensional (2D) selective excitation pulses applied to a subject to be imaged has demonstrated its utility in a number of applications, including spectral localization, real-time NMR cardiac profiling and selective tagging of blood. An acquired NMR signal suffers, however, from artifacts which arise from the discrete manner in which these pulses cover k space, where k space is defined as the spatial frequency domain, obtained by taking the two-dimensional (2D) Fourier transformation of a region in a spatial domain. In particular, pulses derived from spiral k-space trajectories yield concentric ring artifacts surrounding a central excitation region. The spacing between artifacts is inversely proportional to the spacing between adjacent cycles of the spiral. These artifacts may consequently be pushed out to larger radii, and thus out of the imaging region, by increasing the density of sampling of k space. This, however, requires more highly oscillating and therefore longer gradient waveforms, resulting in compromised speed of imaging and bandwidth.
A slice selective presaturation pulse sequence was described in Spatial Presaturation: A Method for Suppressing Flow Artifacts and Improving Depiction of Vascular Anatomy in MR imaging, by J. P. Felmlee, et al., Radiology 164, 559 (1987). This method was employed for saturating NMR signals from flowing blood outside a region of interest thereby reducing blood flow artifacts in an NMR image. This method has been adapted to eliminate the innermost sampling-ring artifact from 2D `pencil excitation` waveforms in NMR cardiac imaging, as described in Rapid NMR Cardiography with a Half-echo M-mode Method, by C. J. Hardy, et al., J. Comput. Assist. Tomogr. 15, 868 (1991). However, this presaturation pulse sequence requires selective excitation and dephasing of magnetization from two separated slabs of tissue in one direction, followed by the same procedure for two slabs of tissue in an direction orthogonal to the first two slabs, and consequently is too long (approximately 17 ms) to be used with some imaging techniques.
It is therefore desirable to provide a method for suppressing excitation artifacts from 2D selective excitation pulses, which is compatible with imaging techniques requiring short imaging pulses, and does not exceed limits for RF power deposition in living subjects.